Retroreflective articles that return back incident light toward the light source are well known. They have been widely used as a reflector of traffic or commercial signs, such as traffic signs, regulating signs, informative signs, and construction signs, or a reflector of visible, laser or infrared light reflective sensors.
In particular, retroreflective articles having a retroreflective element layer composed of an array of a large number of total internal reflective, cube corner retroreflective elements have recently been broadening their application because of their higher retroreflection efficiency, brighter colors for higher visibility, and capability of being backlighted and transmitting the backlight as compared with retroreflective articles using glass microspheres.
Several techniques have been disclosed with respect to the structure of retroreflective articles using the total internal reflective, cube corner retroreflective elements. For example, JP 46-25177B (Patent Document 1) discloses a retroreflective monolithic product of a transparent material having a smooth front surface and a rear surface having a plurality of retroreflecting units each having a set of three mutually perpendicular facets, the rear surface having transparent septa, the septa intersecting to define and isolate a plurality of polygonal cells each from the other, each cell having an area not greater than about six square inches and containing at least three of the cube-corner retroreflecting units, the septa having free edges positioned rearward of the front surface at least as far as the cube-corner retroreflecting units, which free edges permit the structure to be affixed to a backing to hermetically seal the cells individually, and the combined area of the septa being not greater than the combined area of the retroreflecting units.
JP 2001-33609A (Patent Document 2) discloses cube corner retroreflective sheeting comprising a combination of hermetically sealed cells of at least two different kinds, each cell being formed of a light transmitting prism layer, an air layer on the back side of the prism layer, and a binder layer on the back side of the air layer, the prism layer being composed of a light transmitting holding layer having a substantially smooth surface on the light incident side thereof, a face of a prism assembly formed of an array of close packed cube corner retroreflective elements on the back of the holding layer, and a septum integral with the retroreflective elements, the septum sticking beyond the apices of the cube corner elements and surrounding the face of the prism assembly, the top ends of the septum and the binder layer being connected to each other so that the air layer is defined by the septum, the prism assembly face, and the binder layer, wherein the cube corner retroreflective sheeting contains a prism layer comprising retroreflective elements whose optical axis is tilted relative to the vertical line of the light incident side surface and/or a prism layer comprising retroreflective elements of which at least one prism apex angle (the angle at the point of intersection of two facets out of three making the retroreflective element) is slightly deviated from 90 degrees.
The total internal reflective cube corner retroreflective elements used in these techniques are designed to be in contact with an air layer to exhibit retroreflective properties. With respect to a retroreflective article having a retroreflective element layer formed of an array of a large number of such retroreflective elements, it is unknown to provide a layer other than a binder layer on the back side of the retroreflective element layer.
U.S. 2005/0221042A1 (Patent Document 3) discloses retroreflective laminate sheeting having a viewing surface comprising a retroreflective layer having a first cap-Y value of a viewing surface side thereof, and a plurality of discrete pigmented indicia disposed on the viewing surface side of the retroreflective layer, wherein the pigmented indicia define a second cap-Y value of the viewing surface of the sheeting, the second cap-Y value being less than the first cap-Y value.
The technique disclosed consists in adjusting the cap-Y value by the provision of pigmented indicia on the surface of the retroreflective element layer, giving no suggestion of providing a separate layer on the back side of the retroreflective element layer.    Patent Document 1:JP 46-25177B    Patent Document 2:JP 2001-33609A    Patent Document 3:U.S. 2005/0221042A1